The Namib desert is covered with regular patterns of bare circles whose origin is fiercely debated by researchers – but it now seems both leading explanations may be right.
One camp claims the empty patches, known as fairy circles, are created by termites under the soil that clear vegetation in the area around their nests. By making the soil porous, the argument goes, they establish permanent reservoirs of rainwater 50 centimetres below the surface, which sustains them and the surrounding ecosystem.
An alternative idea is that the circles are explained by plants competing for water. Plants help their nearest neighbours by creating shade and maintaining water on the soil’s surface, but hinder those further away by growing long roots that extract water from the soil.
The water competition theory can explain the regular patterns neatly, but hasn’t been proven in any test, says Corina Tarnita of Princeton University. Meanwhile, the termite theory is backed up by observations of termite nests in the circles, but couldn’t explain why the patterns are so regular.
“Each one was bringing what we thought were convincing arguments,” she says.
So Tarnita and her colleague Rob Pringle turned to computer models. First they created one to investigate whether termites could end up with regular spacing between their nests.
Termites forage in a circular area around their nest. When they encounter a smaller termite colony, they destroy it and take over the territory.
But when two colonies of a similar size face off, neither can overcome the other and they establish a border.
The model showed that this competition between termite colonies can lead to a regular honeycomb pattern, with each colony surrounded by six neighbouring colonies. As well as the Namibian fairy circles, Tarnita and Pringle say this pattern can be seen in termite colonies in Arizona, Brazil, Kenya, Mozambique and Australia.
The pair then wondered what would happen if plant competition for water was also at work. “Why does it have to be one or the other?” asks Tarnita.
Their model predicted that there would be two patterns: a large-scale pattern of bare circles, created by termites, and a smaller pattern in the vegetation between the circles, resulting from competition for water.
When they went to Namibia to see the fairy circles for themselves, they saw these smaller circles, about 20 centimetres in diameter and spaced 20 centimetres apart. They hadn’t been reported before.
“The fairy circles have drawn so much attention, people haven’t paid attention to how the vegetation looks between the circles,” says Tarnita. Their observations match the model’s prediction, suggesting the two mechanisms are acting simultaneously.
Norbert Jürgens of the University of Hamburg, Germany, a long-time supporter of the termite theory, is pleased that the new study supports their involvement. “It’s now a balanced discussion,” he says.
But not everyone is convinced. Last year, Stephan Getzin at the Helmholtz Center for Environmental Research in Leipzig, Germany, reported fairy circle-like patterns in the Australian outback just like those found in Namibia.
Getzin says there are no termites in some areas of Namibia and Australia where circles are found, and in others there is no correlation between the circles and termites. “Logically, if there are fairy circles without the presence of termites, the termite theory cannot be considered as a strong explanation for the phenomenon,” he says.
Jürgens says in a decade of research, he has never seen fairy circles without at least some indicators that termites are present, and offers to assist anyone who can’t find them. “I’m happy to help,” he says.
It seems the turf war isn’t over yet.